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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Transition from steady to unsteady Plinian eruption column: the VEI 5, 4.6 ka Fogo A Plinian eruption, Sao Miguel, Azores
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Transition from steady to unsteady Plinian eruption column: the VEI 5, 4.6 ka Fogo A Plinian eruption, Sao Miguel, Azores

机译：从稳定的普林尼尼火山爆发到不稳定的爆发：VEI 5，4.6 ka Fogo一次普林尼埃火山爆发，圣米格尔岛，亚速尔群岛

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摘要

The 4.6 ka Fogo A trachytic Plinian eruption on Sao Miguel Island, Azores, Portugal, is composed by four main phases: 1) Initial phreatomagmatic activity which emplaced thin surge deposits with accretionary lapilli; 2) fully buoyant steady Plinian activity which generated a prominent trachytic basal fall-out deposit; 3) partial collapsing activity, which led to the emplacement of two trachytic-trachydacitic intra-plinian ignimbrites (pink and black ignimbrites) and associated surge deposits, inter-stratified within two fall-out deposits; 4) vent widening and fully collapsing activity which caused the emplacement of the climactic brown trachydacitic ignimbrite. The deposits of these four main phases have been organised into three members FGA(1), FGA(2) and FGA(3), sub-divided into 15 stratigraphic or emplacement units. The fluctuations from a steady to unsteady Plinian eruption column parallel the change in composition of the magma from trachytic to trachydacitic. Although minimal, the change in composition resulted in a marked change in colour of the juvenile clasts from white at the base (first phase) to pink and light-dark grey banded and black (second and third phases) and to dark brown (fourth phase). Density analysis of juvenile clasts revealed that the change in composition did not significantly affect the density of the juvenile pumice pyroclasts, the vesicularity of which increases from 70% to 85% up through the sequence. The pink and the black intra-plinian ignimbrites were emplaced only, within the narrow paleo-valleys of the southern flank with maximum thicknesses of 9 m and 5 m respectively, transitioning onto adjacent topographic highs where thin veneer surge layers were deposited. The fall-out deposits interbedded between the ignimbrites lack basal reverse grading, indicating that the eruption column continued to be sustained during the partial collapses generating the intra-plinian ignimbrites. The climactic dark brown ignimbrite was emplaced radially with a maximum thickness of >40 m. The absence of oxidation surfaces or evidence of reworking within the Fogo A deposits, indicates that there were no time breaks during the different phases of the eruption. The on-land bulk volume of the three ignimbrites has been estimated around 3.2 km(3), while the volume of the intra-plinian fall-out deposits is similar to 0.003 km3. These bulk volume estimations, summed with the basal fall-out volume calculated by Walker and Croasdale (1970), makes a total bulk volume for the Fogo A deposits of 4.4 km(3) indicating a Volcanic Explosively Index of 5, higher than previously estimated. This result is fundamental for the forecasting a possible scenario for a future eruption. (C) 2015 Elsevier B.V. All rights reserved.
机译：葡萄牙亚速尔群岛圣米格尔岛上的4.6 ka Fogo A滑行普利尼星喷发由四个主要阶段组成：1）最初的岩浆活动，将稀薄的潮涌沉积物与增生的lapilli结合在一起； 2）充分浮力的稳定的Plinian活动，产生了明显的曲折基底沉降物； 3）局部塌陷活动，导致两个特拉西—特拉西达克质的p系内部火成岩（粉红色和黑色火成岩）和相关的涌动沉积物在两个沉降沉积物中分层。 4）通风孔的扩大和充分塌陷的活动，导致了高空棕色菱形钙钛矿的火成岩。这四个主要阶段的沉积物已组织为三个成员FGA（1），FGA（2）和FGA（3），细分为15个地层或位置单位。从稳定的Plinian爆发柱到不稳定的Plinian爆发柱的涨落与岩浆成分从曲折到曲曲的变化平行。虽然组成变化很小，但导致幼团的颜色发生了显着变化，从基部的白色（第一阶段）变为粉红色和浅暗灰色带状，黑色（第二和第三阶段）以及深棕色（第四阶段） ）。幼虫碎屑的密度分析显示，组成的变化并未显着影响幼虫浮石碎裂质的密度，其整个囊泡的囊泡度从70％增至85％。粉红色和黑色的lin系内部火成岩仅在南翼的狭窄古河谷中布设，最大厚度分别为9 m和5 m，过渡到相邻的地形高点，在那里沉积了薄的单板涌动层。夹在火成岩之间的沉积物缺乏基性的逆向分级，这表明喷发柱在部分塌陷中一直保持着，从而产生了lin前内部的火成岩。沿径向放置高深暗褐色火成岩，最大厚度> 40 m。 Fogo A沉积物中没有氧化表面或没有返工迹象，表明在喷发的不同阶段没有时间中断。据估计，这三个火成岩的陆上散装体积约为3.2 km（3），而lin内沉积物的体积约为0.003 km3。这些体积估计值加上Walker和Croasdale（1970）计算的基础沉降量，使Fogo A矿床的总体积为4.4 km（3），表明火山爆发指数为5，高于先前的估计。 。该结果对于预测未来喷发的可能情况至关重要。 （C）2015 Elsevier B.V.保留所有权利。

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